[XeTeX] \greeknumeral behaves differently from \roman under xgreek/polyglossia

[Thanos D. Papaïoannou]

Dear all—but especially Apostolos,
In trying to redefine section numbers so that they display in greek numerals via
	\renewcommand{\thesection}{\Greeknumeral{section}},
where \Greeknumeral refers to xgreek's/polyglossia's package of that
name, I received the common error
	! Missing number, treated as zero.
	<to be read again>
What is the canonical way to redefine section numbers under
xgreek/polyglossia? Attached please find an example which reproduces
the issue.

Thanks!,
Thanos

--------------------example.tex---------------------------
\documentclass[11pt]{article}

% hyphenation
\usepackage{polyglossia}

% xetex and fonts
\usepackage{fontspec, xunicode, xltxtra}
\setromanfont[
Mapping=tex-text,
Numbers=OldStyle,
Ligatures=Common,
]{Arno Pro}
\setsansfont[
Mapping=tex-text,
Scale=MatchLowercase,
]{Myriad Pro}
\setmonofont[
Scale=MatchLowercase,
]{American Typewriter}

\setdefaultlanguage{greek}
\renewcommand{\thesection}{\Greeknumeral{section}}

\begin{document}
\section{}

\end{document}